1. Field of the Invention
The present invention relates to a perforation detector for use in a camera for detecting control signals used in advancing and positioning a photographic film.
2. Related Art
In a conventional ISO 135-type photographic film, eight perforations are formed in either side of each exposure frame or picture frame thereof, so that the photographic film may be advanced by one-frame by transporting the photographic film an amount corresponding to eight perforations. To count the perforations of the photographic film, a photo-sensor of reflection type or transmission type is convenient.
Because the one-frame advancing, that is, the positioning of each exposure frame merely effects the spacing between the exposure frames, and the spacing may vary in conventional cameras, it has not been necessary to control a start and stop of the film advancing with a high accuracy. Therefore, it has been sufficient to count the number of photo-electric pulses which are outputted from the photo-sensor, one for each perforation.
Recently, a camera having a magnetic recording head has been proposed for use with a photographic film that has a magnetic recording layer for allowing the recording of various data thereon during the film advancing. Because the data is intended to be utilized for printing, it is necessary to exactly correlate the data recording position to each exposure frame.
It is also known in the art to provide a magnetic recording layer along one lateral side of the exposure frames on a photographic filmstrip, and to provide the perforations along the other lateral side of the exposure frames, allocating one or two perforations to each exposure frame. Thereby, it becomes possible to exactly determine the position of the exposure frame as well as the data recording position for each exposure frame with reference to the one or two perforations.
To determine the data recording position with reference to the perforation, the camera is required to precisely determine the position of the perforation. However, as described above, the perforation detector of the conventional camera can merely detect the number of the perforations passing by the detector. To determine the position of the perforation precisely, it is desirable to detect a leading or trailing margin of each perforation when the margin passes by a predetermined detection position.
The margin of the perforation should theoretically be determined based on the curve of photo-electric signal outputted from a light receiving element of a photo-sensor. However, because the level of the photo-electric signal may be change in accordance with the original difference between the individual photo-sensors. Moreover, because of debris, or the like accumulating on the light projection surface and the light reception surface of the photo-sensor, the output level of the photo-sensor is unavoidably lowered with time.
In the case of a reflection type photo-sensor, a reflection plate is disposed on the opposite side of the photographic film from the light emitting element and the light receiving element, to reflect light passing through the perforation toward the light receiving element to output a distinctively high level signal compared with an output signal detected for light reflected from a non-perforated portion of the photographic film. Therefore, the same problem as above would occur when the reflectance of the reflection plate is lowered with time or for other reasons.